1. Field of the Disclosure
The present disclosure relates to processes of forming electronic devices, and more particularly to processes of forming electronic devices including control gate electrodes, semiconductor layers, and select gate electrodes.
2. Description of the Related Art
Split gate nonvolatile memory cells can be formed such that the surface of the substrate is higher at a region underlying the select gate electrode as compared to another region underlying the control gate electrode. The difference in elevation helps to provide ballistic injection of charge carriers into the charge storage media used for the nonvolatile memory cells, for example, silicon nanocrystals.
During formation of such a memory cell, a select gate dielectric layer and a select gate electrode are formed before forming a charge storage stack and a control gate electrode. The charge storage stack can include a gate dielectric layer, silicon nanocrystals, and a capping dielectric layer. The step formed in the substrate is caused by removing the select gate dielectric layer from an exposed region not covered by the select gate electrode and then by thermally oxidizing the substrate to form the control gate dielectric layer. After forming the remainder of the charge storage stack and the control gate electrode, a set of the silicon nanocrystals lies between the control gate electrode and the substrate (“the substrate set”), and another portion of the silicon nanocrystals lies between the control gate electrode and the select gate electrode (“the select gate set”).
The select gate set of the silicon nanocrystals is problematic. Those silicon nanocrystals can accumulate charge but are difficult to erase. The silicon nanocrystals within the select gate set that lie adjacent to the substrate affect the threshold voltage of the memory cells, and therefore, they still need to be erased even though their erasure is difficult.
Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the invention.